An ultrasonic welding method and an ultrasonic welding device are provided to join a first member having a substantial pipe shape and a second member at a joining surface by pressure-applying and vibration-exciting. In an arranging process, each of a first horn member and a second horn member is slantways arranged in such a manner that a part of a pressure-applying surface thereof which is nearer to a division surface thereof becomes nearer to the joining surface, according to deformation in a joining process. Thus, in the joining process, the pressure-applying surfaces are disposed substantially parallel to the joining surface due to a pressure-applying reaction force, so that the first horn member and the second horn member can be pressed against each other at the division surfaces thereof.